National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Effects of dry tropical forest fragmentation on the reproductive success and genetic structure of the tree Samanea saman.

Abstract: Tropical trees are particularly vulnerable to forest fragmentation because of their low densities, self-incompatibilty system, and high rates of outbreeding. Forest fragmentation is likely to decrease gene flow, increase endogamy, and eventually produce a high differentiation among remnant populations. Our objectives were to evaluate the effect of forest fragmentation on the reproductive success, progeny vigor, and genetic variation of the tropical dry-forest tree Samanea saman. We conducted our study in the dry forest of Costa Rica and compared two tree conditions to evaluate forest fragmentation: (1) isolated, individual trees more than 500 m from the nearest conspecific and surrounded by agricultural fields, pastures, or small remnant forest patches and (2) trees in continuous populations of 10 or more individuals per hectare and surrounded by undisturbed forest. We compared the probability of natural pollination, seed production, genetic variation, and progeny vigor of trees in isolation and in continuous populations. The probability of flowers receiving more than 23 pollen tubes at the base of styles (the flowers more likely to produce a mature fruit) was significantly greater for trees in continuous populations (2.1%) than for trees in isolation (0.4%). However, flowers from the two tree conditions received similar pollen loads on their stigmas. Fruits of trees from continuous populations produced similar numbers of seeds as isolated trees did and had a similar probability of seed abortion. The probability of seed predation by bruchid beetles was significantly greater in continuous populations (30%) than in isolated trees (20%). The number of undamaged seeds ( potentially viable) was similar for both tree conditions. A genetic analysis of progeny showed that levels of genetic diversity in trees in isolation and in continuous populations were comparable. The effective self-fertilization rate and inbreeding coefficient of the progeny were slightly higher for isolated trees than for trees in continuous populations. In addition, there was more genetic similarity in the progeny of isolated trees, within and between fruits. Seeds produced by different fruits within a tree were more likely to be related in an isolated tree than in a tree from continuous populations. Seeds produced by trees from continuous populations were more likely to germinate and to produce greater leaf area and biomass as seedlings than progeny from isolated trees. We concluded that the fragmentation of tropical dry forests affects the genetic variation and vigor of S. saman progeny. Isolated trees showed high reproductive capacity, however, in spite of their habitat condition. Resumen: Los arboles tropicales son particularmente vulnerables a la fragmentacion de bosques debido a su baja densidad, su sistema de auto-incompatibilidad y altas tasas de exogamia. La fragmentacion del bosque posiblemente decrece el flujo de genes, incrementa la endogamia y eventualmente produce una alta diferenciacion entre las poblaciones remanentes. Nuestros objetivos fueron evaluar el efecto de la fragmentacion del bosque sobre el exito reproductivo, el vigor de la progenie y la variacion genetica del arbol de bosque tropical seco Samanea saman. Realizamos nuestro estudio en el bosque tropical seco de Costa Rica y comparamos dos condiciones de los arboles para evaluar la fragmentacion del bosque: (1) individuos aislados mas de 500 m del individuo conespecifico mas cercano y rodeados por campos agricolas, ganaderos o pequenos fragmentos de bosque y (2) arboles en poblaciones continuas de 10 o mas individuos por hectarea y rodeados de bosque no perturbado. Comparamos la probabilidad de la polinizacion natural, produccion de semillas, variacion genetica y vigor de la progenie en arboles aislados y en poblaciones continuas. La probabilidad de que las flores reciban mas de 23 tubos de polen en la base de los pistilos ( las flores con mayor probabilidad de producir un fruto maduro) fue significativamente mayor para arboles en poblaciones continuas (2.1%) que en arboles aislados (0.4%). Sin embargo, flores en arboles en las dos condiciones recibieron similares cargas de polen en sus estigmas. Los frutos de arboles de poblaciones continuas produjeron similares numeros de semillas y tuvieron similar probabilidad de abortar que los de arboles aislados. La probabilidad de depredacion de semillas por escarabajos bruquidos fue significativamente mayor en las poblaciones continuas (30%) que en arboles aislados (20%). El numero de semillas sin dano ( potencialmente viables) fue similar en arboles en las dos condiciones. El analisis genetico de la progenie mostro que los niveles de diversidad genetica de arboles aislados y en poblaciones continuas fueron comparables. La tasa de autofecundacion efectiva y el coeficiente de endogamia de la progenie fue ligeramente mayor en arboles aislados que en los de poblaciones continuas. Adicionalmente, hubo mayor similitud genetica en la progenie de arboles aislados, dentro y entre frutos. Semillas producidas por distintos frutos del mismo arbol tuvieron mas probabilidad de relacion que los de poblaciones continuas. Semillas producidas por arboles en poblaciones continuas tuvieron mayor posibilidad de germinar y producir mayor area foliar y biomasa que las plantulas de la progenie de arboles aislados. Concluimos que la fragmentacion de bosques tropicales secos afecta la variacion genetica y el vigor de la progenie de S. saman. Sin embargo, los arboles aislados tuvieron una alta capacidad reproductiva a pesar de la condicion de su habitat.

Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures

Abstract: We report a high-yield route to two-dimensional arrays (<400×400 μm2) of aligned C49Nx (x ⩽ 1) nanofibers (<100 nm o.d.; <60 μm length), by pyrolyzing mixtures of ferrocene and melamine at 950–1050 °C under an Ar flow. The fibers exhibit unusual interlinked stacked-cone morphologies, ascribed to the presence of nitrogen. High-resolution electron energy-loss spectroscopy of the individual fibers reveals a 2% nitrogen content with ionization energies mainly at ∼400.9 eV, corresponding to N bonded to three C atoms within a hexagonal framework. The nanofibers may be useful for the economic fabrication of field emission sources and robust composites.

Arsenic and fluoride in the groundwater of Mexico

Abstract: Concentrations of arsenic and fluoride above Mexican drinking water standards have been detected in aquifers of various areas of Mexico. This contamination has been found to be mainly caused by natural sources. However, the specific processes releasing these toxic elements into groundwater have been determined in a few zones only. Many studies, focused on arsenic-related health effects, have been performed at Comarca Lagunera in northern Mexico. High concentrations of fluoride in water were also found in this area. The origin of the arsenic there is still controversial. Groundwater in active mining areas has been polluted by both natural and anthropogenic sources. Arsenic-rich minerals contaminate the fractured limestone aquifer at Zimapan, Central Mexico. Tailings and deposits smelter-rich fumes polluted the shallow granular aquifer. Arsenic contamination has also been reported in the San Antonio–El Triunfo mining zone, southern Baja California, and Santa Maria de la Paz, in San Luis Potosi state. Even in the absence of mining activities, hydrogeochemistry and statistical techniques showed that arsenopyrite oxidation may also contaminate water, as in the case of the Independencia aquifer in the Mexican Altiplano. High concentrations of arsenic have also been detected in geothermal areas like Los Azufres, Los Humeros, and Acoculco. Prevalence of dental fluorosis was revealed by epidemiological studies in Aguascalientes and San Luis Potosi states. Presence of fluoride in water results from dissolution of acid-volcanic rocks. In Mexico, groundwater supplies most drinking water. Current knowledge and the geology of Mexico indicate the need to include arsenic and fluoride determinations in groundwater on a routine basis, and to develop interdisciplinary studies to assess the contaminant’s sources in all enriched areas.

Epidemiology of cervical cancer.

Abstract: In 1985, 7.6 million cases of cancer were diagnosed in 24 different regions all over the world (1). Globally, cervical cancer is the fifth most frequent malignancy and second among women, with an annual estimate of 471,000 new cases and 213,000 deaths. The age-adjusted incidence rates for developed countries are 7.6 to 11.8 per 105 and for developing countries, 17.6 to 46.8 per 105 (1,2). This variation in the incidence of cervical cancer is also present within countries, where rural and poor areas have the highest rates.

CLB19, a pentatricopeptide repeat protein required for editing of rpoA and clpP chloroplast transcripts.

Abstract: Summary RNA editing changes the sequence of many transcripts in plant organelles, but little is known about the molecular mechanisms determining the specificity of the process. In this study, we have characterized CLB19 (also known as PDE247), a gene that is required for editing of two distinct chloroplast transcripts, rpoA and clpP. Loss-of-function clb19 mutants present a yellow phenotype with impaired chloroplast development and early seedling lethality under greenhouse conditions. Transcript patterns are profoundly affected in the mutant plants, with a pattern entirely consistent with a defect in activity of the plastid-encoded RNA polymerase. CLB19 encodes a pentatricopeptide repeat protein similar to the editing specificity factors CRR4 and CRR21, but, unlike them, is implicated in editing of two target sites.